1. Technical Field of the Invention
The present invention relates generally to a starter designed to start engines such as automotive internal combustion engines, and more particularly to such a starter equipped with a planetary gear speed reducer and a shock absorber working to absorb impact torque acting on the starter which will arises upon cranking of the engine.
2. Background Art
Japanese Patent First Publication No. 2008-223572, assigned to the same assignee as that of this application, discloses a starter equipped with a multi-disc type shock absorber which includes a plurality of rotary discs formed integrally with an internal gear of a planetary gear speed reducer, a plurality of stationary discs each of which is disposed adjacent two of the rotary discs, and disc springs serving to exert pressure on the stationary and rotary discs to develop mechanical friction therebetween.
When a degree of torque greater than or equal to slip torque preset for the rotary discs is exerted on the rotary discs through the internal gear, it will cause the rotary discs to slip on the stationary discs against the friction between the rotary discs and the stationary discs, thereby permitting the internal gear to rotate to absorb the impact torque acting on the planetary gear speed reducer.
The shock absorber is equipped with a case which has formed on an inner periphery of an open end thereof an internal thread into which a nut with an external thread is fastened. The nut is placed in contacting abutment with the disc springs. An initial pressure or load exerted by the disc springs on a stack of the stationary and rotary discs is adjusted as a function of the amount by which the nut is tightened to set a degree of torque (i.e., the slip torque) which will induce the internal gear to slip.
Specifically, the nut of the shock absorber is designed to serve an important role to determine the slip torque which initiates the slippage of the internal gear.
The shock absorber, however, has the following disadvantage.
When the shock absorber is being assembled, the disc spring may be biased in orientation thereof and experience to have an outer periphery caught by the internal thread of the case. In such an event, the pressure, as produced by tightening the nut, will concentrate at portions of the disc spring and the internal gear caught by each other, which leads to a concern about a lack of application of elastic pressure of the disc springs to the stack of the stationary and rotary discs. Additionally, when ridges of the internal thread are squashed, it will result in a lack of tightening the nut to exert the pressure on the stack of the stationary and rotary discs. This leads to a decrease in the slip torque of the internal gear and, in the worst case, an instability in starting the engine.